Prevalence of anti-Fab antibodies in patients with autoimmune and infectious diseases

Serologic Diagnosis of Rheumatoid Arthritis

Accurate diagnosis of inflammatory arthritides remains a challenge because of substantial clinical overlap. To achieve a granular classification for informing clinical decisions, numerous potential serologic biomarkers have been identified. Rheumatologists have settled on rheumatoid factor and anti-citrullinated protein antibodies for the diagnosis of rheumatoid arthritis (RA) based on specificity and sensitivity and their ability to be integrated into clinical algorithms. These biomarkers should be interpreted in their specific clinical context. This article discusses the serologic basis for the diagnosis of RA, how these biomarkers have framed conceptualization of the pathogenesis of RA, and the inherent limitations in their use.

Beyond citrullination: other post-translational protein modifications in rheumatoid arthritis

The presence of autoantibodies is one of the hallmarks of rheumatoid arthritis (RA). In the past few decades, rheumatoid factors (autoantibodies that recognize the Fc-tail of immunoglobulins) as well as anti-citrullinated protein antibodies (ACPAs) have been studied intensively. ACPAs recognize post-translationally modified proteins in which the amino acid arginine has been converted into a citrulline. More recently, other autoantibody systems recognizing post-translationally modified proteins have also gained attention, including autoantibodies recognizing fragmented immunoglobulin (anti-hinge antibodies), autoantibodies recognizing acetylated proteins and autoantibodies recognizing proteins that are modified by adducts formed under oxidative stress. In particular, detailed insights have been obtained on the presence and properties of autoantibodies recognizing carbamylated proteins, commonly called anti-carbamylated protein (anti-CarP) antibodies. In this Review, we summarize the current knowledge relating to these emerging autoantibodies that recognize post-translationally modified proteins identified in RA, with an emphasis on anti-CarP antibodies.

Protein-based matrix interferences in ligand-binding assays

An adequate bioanalytical support for a typical biotherapeutic requires a number of assays, including those to measure drug concentration and to assess induction of specific immune responses. Ligand-binding assays are the most commonly used platform in bioanalysis of biotherapeutics. Ligand-binding assays are frequently designed to detect appropriate analytes in complex biological matrices with limited or no sample pretreatment steps. The complex composition of the test matrix is highly diverse and varies from normal to disease populations. Additional post-treatment changes are often observed, including induction of antidrug antibodies. Due to potential interaction of biological matrix components, for example, rheumatoid factors, heterophilic antibodies and human anti-animal antibodies, with the test analyte or assay reagents, ligand-binding assays are often subjected to various degrees of matrix interferences that lead to an erroneous under- or over-reporting of the analyte concentration. Impact of various matrix components and practical means designed to mitigate interferences are discussed in this Review.

Antibodies to IgG4 hinge can be found in rheumatoid arthritis patients during all stages of disease and may exacerbate chronic antibody-mediated inflammation

OBJECTIVE

In rheumatoid arthritis (RA), autoantibodies such as anti-citrullinated protein antibodies (ACPAs) develop in response to neoepitopes that are formed under conditions of chronic inflammation. These autoantibodies may subsequently be fragmented by inflammation-associated proteases, leading to the formation of F(ab')2 fragments. The hinge of F(ab')2 fragments can serve as a neoepitope, and so-called antihinge antibodies (AHAs) can be found in RA patients, which might modulate the function of (fragmented) autoantibodies. We undertook this study to investigate the presence and specificities of AHAs in different stages of RA and to study their function.

METHODS

The presence of AHAs was assessed by radioimmunoassay in healthy controls, blood donors who later developed RA, patients with arthralgia, patients with early RA, and patients with established RA. Specificity of the AHAs was analyzed with inhibition assays, and complement-activating ability was studied with a C4b deposition assay.

RESULTS

Antibodies to IgG1 hinge, IgG2 hinge, and IgG4 hinge were detected in patients with established RA, with anti-IgG4 hinge antibodies being most specific (appearing in 1% of healthy controls, 3.8% of blood donors who later developed RA, 13% of arthralgia patients, 19% of early RA patients, and 16% of established RA patients). Anti-IgG4 hinge antibodies were subclass specific and were able to restore C4b deposition by IgG4 F(ab')2 fragments. In patients with arthralgia and patients with early RA, anti-IgG4 hinge antibodies were associated with rheumatoid factors and ACPAs.

CONCLUSION

Anti-IgG4 hinge antibodies are present in RA patients and have low sensitivity but high specificity for RA. Since a significant proportion of ACPAs can be of the IgG4 subclass, the formation of anti-IgG4 hinge antibodies may represent one mechanism of ACPA-mediated inflammation.

Rheumatoid factor revisited

PURPOSE OF REVIEW

Initial studies of the pathogenesis of rheumatoid arthritis focused on the role of rheumatoid factor and immune complex-associated vasculitis and synovitis. Subsequent work has delineated T cell responses, the role of cytokines, chemokines, and the aggressive nature of rheumatoid synovitis. Recent findings underscore the importance of humoral immunity in this entity and are the subject of this review.

RECENT FINDINGS

By the discovery of anti-cyclic citrullinated peptide, anti-RA33, and anti-GPI antibodies in the human and mouse systems, respectively, the impact of humoral autoimmunity in rheumatoid arthritis regained remarkable interest. This review summarizes recent insights into humoral autoimmunity in rheumatoid arthritis in the context of the generation of rheumatoid factors, including B cell activation via toll-like receptors and genetic predispositions that can trigger the induction of rheumatoid arthritis. The generation of rheumatoid factors that can also be found during host defense against infectious agents and under pathologic conditions, such as rheumatoid arthritis, Sjögren syndrome, and hepatitis C-associated mixed cryoglobulinemia after hepatitis C infection is likely the result of genetic predispositions and the intensity of the (primary) immune reaction. Models of the role of rheumatoid factors in health and disease, including related lymphomagenesis, will be discussed.

SUMMARY

In patients with rheumatoid arthritis, the induction of rheumatoid factors can be taken as an indicator of severe disease with a striking involvement of B cell activation. Very recent clinical trials using B cell depletion support the concept that humoral immunity, as evidenced by the production of rheumatoid factors, plays a significant role in the course of the disease.

Musculoskeletal manifestations in cystic fibrosis

Although bone and joint manifestations are common in children with cystic fibrosis (CF), they have received little attention in adults. As compared to healthy individuals, bone mineral density is low, even with calcium intakes greater than 1500 mg/d. Nevertheless, calcium and phosphate levels in blood and urine are often normal, and vitamin D levels vary. Short stature with a low body mass index and central hypogonadism are the rule in these patients. Fractures and kyphosis are often reported. CF arthropathy occurs in 2-8.5% of patients. Arthritis develops, and there may be skin eruptions. Non-steroidal antiinflammatory drug therapy is effective. Hypertrophic osteoarthropathy associated with respiratory failure is present in 2-7% of patients. Rheumatoid arthritis, spondyloarthropathies, sarcoidosis, and amyloidosis have been reported in association with CF. Knee pain due to patellofemoral syndrome, quinolone-induced arthropathy, and mechanical back pain have been described. Rheumatoid factor titers are higher than in healthy controls, particularly in patients with episodic arthritis. No data are available on antiperinuclear factor or antikeratin antibody titers. Tests for antinuclear antibody are usually negative. Circulating immune complex levels and antibodies to heat shock proteins may be elevated. Antineutrophil cytoplasmic antibody of the bactericidal/permeability-increasing protein (BPI) or azurocidin (AZ) type has been reported, often in high titers (up to 40%).

Rheumatoid factors: host resistance or autoimmunity?

Rheumatoid factors (RFs), autoantibodies that bind to the Fc portion of IgG, are important in the immune response. RF-committed B-cells exist in the circulating lymphocyte pool in a high frequency (approximately 1-2 %) in normal individuals and in patients with pathological conditions associated with the sustained levels of circulating RF, such as rheumatoid arthritis (RA), Sjogren's syndrome (SS), and mixed cryoglobulinemia, associated with hepatitis C virus infection. RFs are induced by many infectious entities (viruses, bacteria, parasites) as a consequence of a secondary immune response to the pathogen, but usually the response is transient. It is likely that RFs play an important role in the host's defense against infection, both at the cellular level, where the RF B-cell can be an antigen presenting cell which can promote the antipathogen response, and at the humoral level, where RFs can contribute to the mopping up of the IgG antipathogen antibodies by contributing to immune complex formation and clearance. There has been much research on RFs in chronic pathological conditions, and the literature pertaining to their origin, structure, binding specificities, and possible roles in disease are discussed. The importance of the host defense, sometimes at the expense of an autoimmune response, is a balance that needs to be considered in light of a possible outcome of health or disease.

New procedure for the detection of complement deficiency by ELISA. Analysis of activation pathways and circumvention of rheumatoid factor influence

A procedure using enzyme-linked immunosorbent assays for the assessment of complement function has been evaluated. The sera investigated were incubated in microtiter plates with solid-phase complement activators. Human polyclonal IgG or monoclonal IgM were used for classical activation pathway assays and Salmonella typhosa lipopolysaccharide (LPS) for alternative activation pathway assays. The analysis focussed on deposition of C9 and properdin as detected with enzyme-conjugated antibodies. In an attempt to avoid spurious results due to rheumatoid factors in patient sera, monoclonal mouse and chicken antibodies were unsuccessfully tested as indicator reagents in the assay with solid-phase IgG. However, the use of solid-phase IgM as an activator completely circumvented the influence of rheumatoid factors. With solid-phase IgG or IgM, properdin deposition occurred in the absence of factor D. A combination of assays is suggested for diagnostic purposes: IgM-coated plates with detection of bound C9 and properdin for the classical pathway and LPS-coated plates with detection of bound properdin for the alternative pathway. The procedure distinguished between defects of the classical activation pathway (C1, C4, C2), the alternative activation pathway (C3, factor B, factor D, properdin) and the terminal components (C5-C9). This analytical approach may be useful for detection of inherited complement deficiency and the assessment of complement function in acquired complement deficiency states.

IL-2 enhances polyclonal IgM but not IgM-rheumatoid factor synthesis by activated human peripheral blood B cells

IgM-rheumatoid factor (RF) is thought to be involved in the pathogenesis of rheumatoid arthritis (RA). Several cytokines are known to regulate immunoglobulin synthesis. In this study the effects of IL-2 on polyclonal IgM and IgM RF synthesis were compared. Cytokines were added to peripheral blood B cells from normal subjects and patients with RA after activation by Staphylococcus aureus Cowan 1 (SAC). The addition of IL-2, but not IL-4 or IL-6, resulted in significant enhancement of IgM synthesis in cultures from both healthy subjects and patients with RA. Similar degrees of enhancement were seen in both peripheral blood mononuclear cell and highly purified B cell cultures. IgM-RF was synthesized after activation in cultures from healthy subjects and spontaneously in cultures from RA patients. In contrast to polyclonal IgM synthesis, IL-2 failed to augment IgM-RF synthesis in cell cultures from either healthy subjects or RA patients. This study demonstrates different effects of IL-2 on IgM and IgM-RF synthesis.